In the measurement of light by photometric instruments, particularly in space laboratory measurement of vacuum ultraviolet light from space in the presence of high intensity visible light from the sun, there is a recognized need for optics which favor reflection of the VUV wavelengths being studied and which reject near ultraviolet and visible light. For these purposes reflectance filters, as compared to the usual transmission filters, have been described by Osantowski and Toft in Applied Optics, Vol. 12, No. 6, pages 1114 and 1115, June, 1973, and also by Hass and Tousey, Journal of the Optical Society of America, Vol. 49, page 593, 1959.
The reflectance filter described by Osantowski and Toft comprised an opaque aluminum film covered by a quarter wavelength of magnesium fluoride over which a semitransparent aluminum film and a final protective magnesium fluoride layer.
It is the object of the present invention to provide a reflectance filter which has improved reflectance well into the vacuum ultraviolet and improved rejection of light outside the VUV.